Earliest cephalopods

The cephalopods constitute the taxonomic class of molluscs that contains squids, octopuses, cuttlefishes, nautiluses and famous extinct forms such as ammonites and belemnites. The earliest fossils deemed to be true cephalopods, i.e., originating the lineage that gave rise to modern-day forms, are the ellesmeroceridans, which belong to the still-surviving subclass Nautiloidea, and are known from certain fossils dating back as far as the Upper Cambrian Period, approximately 500 million years ago. They possessed small, slightly curled shells containing several internal, sequentially arranged chambers. Some specimens have been obtained from strata in what is today New York State, USA.

Several different genera of ellesmeroceridan have been uncovered and formally described, including Paradakeoceras, Quebecoceras, Levisoceras, Burenoceras and Dakeoceras.

The ellesmeroceridans were the only cephalopods to survive the mass extinction that occurred at the end of the Cambrian Period, and diversified dramatically during the Ordovician Period that followed.